The need for articles produced from nonwoven containing nanofibers has continued to increase. The diameters of nanofibers are generally understood to be less than about 1000 nanometer or one micron. The nanofibers webs are desired due to their high surface area, low pore size, and other characteristics. The nanofibers, also commonly called microfibers or very fine fibers, can be produced by a variety of methods and from a variety of materials. Although several methods have been used, there are drawbacks to each of the methods and producing cost effective nanofibers has been difficult. Therefore, hygiene articles and other disposable consumer products containing nanofibers have not been marketed.
Methods of producing nanofibers include a class of methods described by melt fibrillation. Melt fibrillation is a general class of making fibers defined in that one or more polymers are molten and extruded into many possible configurations, such as co-extrusion, homogeneous or bicomponent films or filaments, and then fibrillated or fiberized into fibers. Nonlimiting examples of melt fibrillation methods include melt blowing, melt film fibrillation, and melt fiber bursting. Methods of producing nanofibers, not from melts, are film fibrillation, electro-spinning, and solution spinning. Other methods of producing nanofibers include spinning a larger diameter bicomponent fiber in an islands-in-the-sea, segmented pie, or other configuration where the fiber is further processed after the fiber has solidified so that nanofibers result.
Melt blowing is a commonly used method of producing fibers. Typical fiber diameters range from 2 to 8 micron. Melt blowing can be used to make fibers with smaller diameters but with considerable changes needed to the process. Commonly, redesigned nozzles and dies are needed. Examples of these include U.S. Pat. Nos. 5,679,379 and 6,114,017 by Fabbricante et al. and U.S. Pat. Nos. 5,260,003 and 5,114,631 by Nyssen et al. These methods utilize relatively high pressures, temperatures, and velocities to achieve the small fiber diameter.
Melt film fibrillation is another method to produce fibers. A melt film tube is produced from the melt and then a fluid is used to form nanofibers from the film tube. Two examples of this method include Torobin's U.S. Pat. Nos. 6,315,806; 5,183,670; and 4,536,361; and Reneker's U.S. Pat. Nos. 6,382,526 and 6,520,425, assigned to the University of Akron. Although these methods are similar by first forming a melt film tube before the nanofibers result, the processes use different temperatures, flow rates, pressures, and equipment.
Film fibrillation is another method of producing nanofibers although not designed for the production of polymeric nanofibers to be used in nonwoven webs. U.S. Pat. No. 6,110,588 by Perez et al., assigned to 3M, describes of method of imparting fluid energy to a surface of a highly oriented, highly crystalline, melt-processed polymer film to form nanofibers. The films and fibers are useful for high strength applications such as reinforcement fibers for polymers or cast building materials such as concrete.
Electrospinning is a commonly used method of producing nanofibers. In this method, a polymer is dissolved in a solvent and placed in a chamber sealed at one end with a small opening in a necked down portion at the other end. A high voltage potential is then applied between the polymer solution and a collector near the open end of the chamber. The production rates of this process are very slow and fibers are typically produced in small quantities. Another spinning technique for producing nanofibers is solution or flash spinning which utilizes a solvent.
Two-step methods of producing nanofibers are also known. A two-step method is defined as a method of forming fibers in which a second step occurs after the average temperature across the fiber is at a temperature significantly below the melting point temperature of the polymer contained in the fiber. Typically, the fibers will be solidified or mostly solidified. The first step is to spin a larger diameter multicomponent fiber in an islands-in-the-sea, segmented pie, or other configuration. The larger diameter multicomponent fiber is then split or the sea is dissolved so that nanofibers result in the second step. For example, U.S. Pat. No. 5,290,626 by Nishio et al., assigned to Chisso, and U.S. Pat. No. 5,935,883, by Pike et al., assigned to Kimberly-Clark, describe the islands-in-the-sea and segmented pie methods respectively. These processes involve two sequential steps, making the fibers and dividing the fibers.
It is desired to produce a uniform nanofiber web with low basis weight made from commonly used polymers, such as polypropylene and polyethylene. Although there is much disclosure on making nanofibers and web, a uniform web at low basis weights and with common polymers have not been produced. Electrospinning is a common way to make nanofibers but not suitable for polyolefins such as polypropylene or polyethylene. Polystyrene can be used in electrospinning but is too brittle and will form beads. Additionally, electrospinning is not a suitable method for high speed production or for in-line processing with other layers for webs. Other methods to made nanofibers have been disclosed but are not controlled enough to make low basis weight uniform webs. A uniform web is required as any type of hole or non-uniformity may create an unacceptable barrier. Therefore, there is a great desire to produce a uniform low basis weight web comprising a significant number of nanofibers.
To produce disposable hygiene articles containing nanofibers that are commercially advantageous, the cost of the nanofibers must be controlled. Equipment cost, process costs, any additional process aids, and polymer costs are all areas where costs can be controlled. Therefore, it is an object of the invention to produce nanofibers which are low in cost.